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Cold Spray Forming Inconel 718
W. Wong,* E. Irissou,** J.-G. Legoux, F. Bernier
Industrial Materials Institute, National Research Council Canada, Boucherville, Quebec, Canada*E-mail: wilson.wong@mail.mcgill.ca; **E-mail: eric.irissou@cnrc-nrc.gc.ca
P. Vo, S. Yue
Department of Mining and Materials Engineering, McGill University, Montreal, Quebec, Canada
S. Michiyoshi, H. Fukanuma
Plasma Giken Co., LTD., Tokyo, JapanSEM and for mechanical tensile testing. The effect of postsprayed heat treatment was also investigated.

Abstract
In this investigation, Inconel 718, a material known to cause
nozzle clogging upon cold spraying, was cold spray formed to
6 mm-thick using the Plasma Giken cold spray system PCS1000. This was made possible due to the novel non-clogging
nozzle material combined with a nozzle water cooling system.
Coatings were as-spray formed using both nitrogen andhelium as the propelling gasses. The resulting microstructures
as well as the corresponding mechanical properties werestudied. In addition, the effect of post-heat treatments was also
investigated. It was found that for a given propelling gas used,the coating porosity level remained relatively similar (about
2.4% for nitrogen and 3.6% for helium) regardless of thecoating treatment (as-sprayed or heat treated). Visualinspection from SEM micrographs showed a higher fraction ofinter-particle metallurgical bonds for nitrogen gas sprayedcoatings heat treated at 1250°C for 1 hour due to somesintering effect. This significantly affected its tensile
properties with an average resulting ductility of 24.7%.

Experimental
Feedstock Powder
Commercially available Inconel 718 powder (Amdry 1718)
manufactured by Sulzer Metco (Troy, USA) was used. The
powder size distribution was measured with a laser diffraction
particle size analyser, the Beckman Coulter Model LS 13 320.
Fig. 1(a) shows the particle size distribution of the Inconel 718
feedstock powder. Note that the powder revealed a narrow
particle size distribution. The mean particle size was 37 ȝm.
Fig. 1(b) shows the spherical morphology of the Inconel 718
feedstock powder, while Fig. 1(c) shows an as- polished crosssection. The latter micrograph indicates that the particles were
non-porous.
Cold Spray Parameters
Coatings were produced with the PCS-1000 (Plasma Giken
Co., Tokyo, Japan) cold spray equipment. The sprayparameters using nitrogen gas include: propelling gas pressure
and temperature of 5 MPa and 1000°C, respectively, a nozzle
standoff distance of 25 mm, a substrate holder rotation speedof 600 rpm, and a gun traverse speed of 10 mm/s. The coatingproduced was approximately 6 mm-thick. Similar sprayparameters were used using helium as the propelling gas; theonly exception was that an inlet gas pressure of 2 MPa wasused instead of 5 MPa. The Inconel 718 powder was cold
sprayed onto 8 cm-diameter aluminium tubes.

Introduction
Cold spray technology has tremendous potential for producing
thick coatings with soft metals such as aluminium and copper(Ref 1-6). However, high critical velocity, residual stresses,
and technical issues such as nozzle clogging have limited thenumber of materials that have been cold spray formed (Ref 710). For example, although the aerospace industry would
consider cold spray forming nickel-based superalloys (i.e.,
Inconel 718) for many potential applications, this material has
been challenging to spray mainly due to nozzle clogging. This
problem has been recently solved in commercial cold sprayequipment by using a non-clogging nozzle material combinedwith a nozzle water cooling system. In this work, Inconel 718
powder was cold sprayed using Plasma Giken PCS-1000
equipment to produce 6 mm-thick coatings on 8 cm-diameteraluminium tubes using both helium and nitrogen as thepropelling gasses. The spray formed parts were sectioned to
produce several samples for material characterisation throughSample Extractions and Heat Treatment Conditions andProcedure
The as-sprayed Inconel 718 coatings were sectioned throughwire-EDM in order to extract tensile test samples(perpendicular to the plane of spray direction). The
dimensions of each tensile test are depicted in Fig. 2 above.
Four tensile test specimens were kept as-sprayed while others
were post-heat treated in a furnace under a 90% argon/10%

243hydrogen atmosphere at 950°C/2 hours, 1010°C/2 hours,
1060°C/2 hours, and 1250°C/1 hour (3 samples for each heat
treatment).

The heat treatment procedure is outlined below. The samples
were placed onto a rectangular metallic stage/holder which
was pre-treated with boron nitride in order to prevent the
samples from adhering onto the stage/holder. Next, thestage/holder was inserted into a glass tube which has two
openings: one end with an opening diameter of 101.6 mm(from which the stage/holder was inserted) and the other end
with a small pinhole opening diameter of 3.5 mm (for purging
air from the glass tube). A specialised cover with athermocouple and a plastic tube for argon gas flow injection
attached to it was then placed and tightened onto the larger
glass tube opening. Then, the argon gas at 6 SCFH (Standard
Cubic Feet Hour) was released into the plastic tube for at least
20 minutes in order to purge the air from the glass tube and tocompletely surround the samples in an argon atmosphere(continuous argon gas flow). An air furnace with an openingof the same dimensions as the glass tube was set to 700°C.
When this temperature was reached, the glass tube waspartially inserted inside the air furnace and was left there
while 10% hydrogen gas was injected into the argon gas flow.
When the gas mixture was completed, the tube was fully
inserted inside the air furnace where the temperature rose at aconstant rate of 10°C per minute until the desired heat
treatment temperature was reached for each set of coating heat
treatment. After the heat treatment was completed, thehydrogen gas was turned off and the glass tube was partiallyremoved from the air furnace in order to allow the remaininghydrogen gas to burn. Meanwhile, the tube was left to cool
down to room temperature. This was performed while
maintaining the continuous argon gas flow atmosphere inside
the glass tube until the glass tube reached room temperature,
after which the heat treated samples were removed.
Metallographic Preparation and Examination
Using standard metallographic preparation procedures, the assprayed and heat treated samples were sectioned with acoolant-assisted diamond wheel, cold vacuum-mounted in an
epoxy resin then ground and polished. Microstructuralobservations were performed using a JEOL 840 scanningelectron microscope (SEM) and a field emission gun scanningelectron microscope, the Hitachi S-4700 (FEG-SEM). Powder
morphology was analysed using FEG-SEM in secondary
electron imaging (SEI) mode, while the as-polished coating
cross-sections were analysed in backscattered electron
imaging (BSI) mode. SEM BSI mode was also used to
conduct coating porosity measurements. These were assessedon the as-polished samples with the use of image analysis
software. A minimum of ten random images were taken andporosity was evaluated for each image.

Figure 1. (a) Feedstock powder size distribution, (b) FEGSEM micrograph depicting the morphology of the spherical
Inconel 718 powder, and (c) As-polished powder crosssections.

Tensile TestingTensile tests were performed in accordance with the ASTM
E8/E8M – 09 standard using an Instron 5582 universal testingmachine (Burlington, Ontario, Canada) with a dynamic load
up to 100 kN at a constant speed of 0.05 mm/sec. All tests
used an extensometer with a gage length of 25 mm.

Figure 2. Dimensions of the tensile test specimens extracted
via wire-EDM (all numbers are in mm).

244

increased fraction of adiabatic shear instability (ASI)
occurrences between inter-particle boundaries. This is shown
schematically in Fig. 5.

Results and Discussion
As-Sprayed and Heat Treated Cold Sprayed Inconel 718
Coating Characterisations
The porosity of the as-sprayed and heat treated coatings arepresented in Table 1 (for both nitrogen and helium propelling
gasses used). The error numbers presented in Table 1
represent the standard error.
Table 1. Coating Porosity of As-Sprayed and Heat TreatedSamples
Coating
Treatment
As-Sprayed
950°C/2hrs
1010°C/2hrs
1060°C/2hrs
1250°C/1hr

Porosity %(N2 Inlet Gas)
2.7 ± 0.4
2.4 ± 0.2
1.9 ± 0.2
2.8 ± 0.2
2.0 ± 0.6

Porosity %(He Inlet Gas)
3.4 ± 0.4
3.3 ± 0.5
3.7 ± 0.3
3.7 ± 0.4
3.8 ± 0.3

Cross-sections of the as-sprayed and heat treated cold sprayed
Inconel 718 coatings for nitrogen and helium propelling
gasses are shown in Figs. 3 and 4, respectively. When
comparing Figs. 3 and 4 from visual inspection, it appears thatthe as-cold sprayed and heat treated coatings using nitrogen as
the inlet propelling gas were slightly denser than those
produced using helium as the propelling gas. Quantitative
analyses further support these observations (see Table 1). For
coatings sprayed with a specific propelling gas (either withnitrogen or helium), the coating porosity level remained
relatively similar regardless of the coating treatment (assprayed or heat treated). In all cases (Figs. 3 and 4), the pore
distributions were relatively homogeneous across the coating
cross-sections. However, closer observation revealed that, forthe coating sprayed with nitrogen as the propelling gas andsubsequently heat treated at 1250°C for 1 hour (Fig. 3(e)), its
microstructure showed a distinct feature: the inter-particle
boundaries were almost invisible compared to all othercoatings investigated. This may perhaps be due to somesintering effect at this high temperature combined with theinitial cold spray conditions.
Impacting particle velocities were measured for the cases ofnitrogen sprayed (787 m/s) and helium sprayed (741 m/s)
coatings. The impacting particle velocity for nitrogen sprayedcoatings was faster than those sprayed with helium, possibly
because: (1) although the inlet gas temperature was set to1000°C in both cases, the inlet gas pressure was significantly
lower when using helium as the propelling gas (2 MPa versus
5 MPa) and (2) the cold spray gun nozzle was not optimised.
Thus, although the impacting particle velocity is usuallyhigher when using helium as the propelling gas (Ref 11-12), in
this specific situation, the particle velocity was faster whenusing nitrogen as the propelling gas. Consequently, higher
particle plastic deformation is expected to occur with an

Figure 3. Cross-sections of cold sprayed Inconel 718 coatings
using nitrogen as the propelling gas: (a) As-sprayed, (b) Heat
treated at 950°C/2hrs, (c) Heat treated at 1010°C/2hrs, (d)Heat treated at 1060°C/2hrs, and (e) Heat treated at
1250°C/1hr.

245

Figure 5. Schematic representation of particle deformation
having higher impacting particle velocity and ASI occurrences(a) as opposed to lower impacting particle velocity and ASI
occurrences (b).
Thus, the as-sprayed coatings using nitrogen as the propellinggas were expected to have an increased fraction of
metallurgical bonding and consequently, a lower coating
porosity level, as also shown from quantitative analyses
through image analysis in Table 1. Following heat treatment at
a high temperature (1250°C for 1 hour), sintering occurred.
The stages of sintering are schematically depicted in Fig. 6.
During the initial stage of sintering, some metallurgicalbonding was already present from ASI during the cold sprayprocess. Neck growth took place when moving from the initial
to the intermediate stage, increasing the metallurgicallybonded area. Finally, pore spherodisations occurred and incertain cases, these pores may be replaced by metallurgicallybonded inter-particle boundaries (Ref 13). Due to the initiallylower coating porosity level in the as-sprayed coatings
produced using nitrogen gas (Fig. 3(a)) as opposed to thehelium gas (Fig. 4(a)), fewer spherodised pores from the final
stage of sintering would be expected for the nitrogen gas
sprayed coatings. This was further confirmed from visualinspection of Fig. 3(e) and Fig. 4(e) at higher magnifications,shown in Figs. 7 and 8, respectively. The red arrows indicate
the spherodised pores after sintering at a high temperature.
Thus, this could explain the dissipation of the inter-particle
boundaries seen in Fig. 3(e) and not in Fig. 4(e).

Figure 4. Cross-sections of cold sprayed Inconel 718 coatings
using helium as the propelling gas: (a) As-sprayed, (b) Heat
treated at 950°C/2hrs, (c) Heat treated at 1010°C/2hrs, (d)Heat treated at 1060°C/2hrs, and (e) Heat treated at
1250°C/1hr.

Figure 6. Schematic representation of the sintering stages.

246

Figure 7. Cross-section of cold sprayed Inconel 718 coatings
using nitrogen as the propelling gas heat treated at 1250°C
for 1 hour.

Figure 9. Tensile test results for as-cold sprayed and heat
treated Inconel 718 coatings using nitrogen as the propellinggas.

Figure 8. Cross-section of cold sprayed Inconel 718 coatings
using helium as the propelling gas heat treated at 1250°C for
1 hour.

Figure 10. Tensile test results for as-cold sprayed and heat
treated Inconel 718 coatings using helium as the propellinggas.

Tensile Strength and Ductility
The resulting observations from the previous Section may
have strong influences on the resulting mechanical propertiesof these coatings. To verify this hypothesis, tensile tests were
performed for all coatings presented in Figs. 3 and 4.
Examples of the resulting engineering stress versus
engineering strain curves are shown in Figs. 9 and 10,
respectively.

As expected, the Inconel 718 coating that was as-cold sprayedusing nitrogen gas and was subsequently subjected to a heat
treatment at 1250°C for 1 hour revealed the highest ductility
by far, with an average engineering strain value of 24.7% fromthree repeats. This ductility has even exceeded certain specificbulk Inconel 718 sheets, strips, and plates that have beenannealed and aged (24.7% versus 11-12%). On the other hand,
the helium gas sprayed Inconel 718 coating that was subjectedto the same heat treatment only showed an average
engineering strain value of 2.2% from three repeats. One
plausible explanation of the low ductility for the helium cold
sprayed coatings would be due to the amount of plastic
deformation (i.e., cold work) prior to the heat treatment. It is
well known that the amount of cold work has a criticalinfluence on the ductility of nickel and nickel alloys (i.e.,
Inconel 718) after annealing heat treatments, which are

From Figs. 9 and 10, it can be clearly seen that regardless ofthe propelling gas used to produce the Inconel 718 coatings, as
the heat treatment temperature increased, the coating became
more ductile (increasing engineering strain). This may be
explained by the better inter-particle (metallurgical) bonding
which resulted from the sintering at the high temperatures.

247

typically performed above 955°C for approximately an hour
(Ref 13). When a low amount of cold work or plastic
deformation is present prior to heat treatment (as is the casewith the helium sprayed coating), full ductility usually cannot
be restored by annealing because of excessive grain growth
due to critical strain, even if the hardness is recovered to thatof the pre-cold worked state (Ref 13). In addition, since thehelium cold sprayed coatings would have a lower fraction of
metallurgically bonded inter-particle boundaries after sinteringthan the nitrogen coatings, this would further reduce thelikelihood of achieving higher ductility. Thus, a minimum
amount of plastic deformation is required to ensure maximum
ductility and softness after annealing heat treatments. Further
investigation on this matter would be required. In addition,
when comparing Figs. 9 and 10, for the nitrogen gas sprayedcoatings, increasing heat treatment temperatures not only
increased the engineering strain, but also increased theengineering stress. However, this was not observed for the
helium gas sprayed coatings, in which increasing heat
treatment temperatures increased the engineering strain but
decreased engineering stress. These issues will be addressed in
the near future along with a more detailed microstructuralinvestigation of the coating cross-sections through chemical
etching.

Acknowledgements
The authors would like to thank Mr. David de Lagrave andMr. Michel Thibodeau from NRC-IMI for metallographic
preparations/testings and microscopy, respectively. Additional
appreciation goes to Mr. Renzhong Huang from Plasma Giken
Co. for cold spray sample preparations and Mr. Mario
Laplume from NRC-IMI for performing sample heat
treatments.
References
1.

V. K. Champagne, The Cold Spray Materials Deposition
Process: Fundamentals and Applications, Woodhead Pub.
Ltd., Cambridge, U.K., 2007
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Pressure Gas Dynamic Spray: Physics & Technology,
Wiley-VCH, Weinhelm, Germany, 2008
3. A. Papyrin, V. Kosarev and S. Klinkov, Cold Spray
Technology, Elsevier Ltd., Amsterdam, Netherlands., 2007
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Investigation of Al-Al2o3 Cold Spray Coating Formation
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Conclusions
The microstructural and mechanical properties of as-sprayedand heat treated cold sprayed Inconel 718 coatings were
investigated. The resulting coatings microstructures andporosity were analysed. In addition, tensile tests were
performed and the results were analysed and discussed. It was
concluded that:(1) In general, coatings sprayed with a specific propelling gas(either with nitrogen or helium) revealed similar coating
porosity levels regardless of the coating treatment (as-sprayedor heat treated).
(2) After heat treatment at 1250°C for 1 hour, the coating
sprayed with nitrogen propelling gas showed a significant
amount of inter-particle metallurgical bonds that was not seenin all other coatings investigated (i.e., coatings heat treated atlower temperatures or coatings sprayed with helium propellinggas). It was hypothesised that this could be due to somesintering effect at this high temperature combined with theinitial cold spray conditions which in turn potentially resulted
in high impacting particle velocities and consequently, a large
fraction of adiabatic shear instability occurrences.
(3) Enhanced inter-particle metallurgical bonds significantly
influenced the resulting mechanical properties of the coatingsuch as ductility. From the tensile test results, the highest
ductility was found to be at about 24.7% for the nitrogen gas
sprayed Inconel 718 coating which was heat treated at 1250°C
for 1 hour. The corresponding ultimate tensile stress was about
763.6 MPa, which is approximately 70% of that of the bulk
strength.

248